[flang] Port to macOS, where mktemp doesn't have -p.

Original-commit: flang-compiler/f18@bd698aee68
Reviewed-on: https://github.com/flang-compiler/f18/pull/522

flang/test/semantics/test_errors.sh

@@ -30,7 +30,7 @@ case $1 in
 esac
 [[ ! -f $src ]] && echo "File not found: $src" && exit 1
 
-temp=`mktemp -d -p .`
+temp=`mktemp -d ./tmp.XXXXXX`
 [[ $KEEP ]] || trap "rm -rf $temp" EXIT
 
 log=$temp/log